Energy exchange processes between two working media of different total pressure and temperature play a key role in the field of aeropropulsion. Generally, rotating fluid flow machines such as turbines, compressors, and fans are employed to perform the energy exchange processes in aeropropulsion systems. However, a great deal of effort has been spent to enable the use of energy exchange processes not employing rotating machinery. These are processes in which the two working media are brought in direct contact with each other, e.g., direct energy exchange processes. Typical representatives are ejectors of the steady flow or crypto-steady type, pressure exchange processes, pulse or ram jets, and others. The significance of the direct energy exchange processes lies in a substantial number of advantages over rotating machinery, namely, structural simplicity, low weight, low cost, high reliability because of the absence of high speed machine elements, use of ultra-high temperature materials including nonmetallic materials (due to the absence of centrifugal stresses), and use of nonstrategic materials and materials resulting in smaller radar cross-sections.
Direct momentum (or energy) exchange processes offer the possibility of achieving very compact lightweight engine structures. The important operational and performance characteristics of such devices are to a large measure a direct consequence of the absence of rotating machinery. Extreme short response time of power output to changes in fuel input is available due to the absence of the moment of inertia of a turbomachinery rotor. The operational boundaries of the engine are not determined by temperature-stress limitations of rotating components as is the case in gas turbine engines, but by internal Mach numbers and temperature limitations of non-moving combustor components. Therefore, the corrected speed of the engine can be kept constant over a much wider range of flight Mach numbers and altitudes than is possible for a gas turbine engine. Also, excellent storability is possible due to the absence of bearing and lubrication systems, which is very important for missile engines.
Current direct energy exchange processes can be grouped into two major categories, (a) those which use unsteady flow processes such as stock tubes, pulse jets, pressure exchangers, and unsteady or crypto-steady ejectors; and (b) steady flow processes such as continuous flow ejectors used a pumps, thrust augmentors, and other applications. The unsteady flow direct momentum exchange processes, when used as a primary propulsion system, have a relatively low overall efficiency and a low power density in comparison to turbomachinery systems, and in some cases have very severe noise and vibration problems which can be more destructive than the high stresses in rotating machines.
The current steady flow ejector systems, while simple and elegant in structure, cannot be used as primary components in propulsion systems. Their potential applicability is limited to augmentors of mass flow and thrust of conventional or existent primary propulsion systems. Several independent studies have shown that ejector-thrust augmentation ratios are highest at stillstanding and decrease to zero around a flight Mach number of 1. From this point on the thrust augmentation ratios, through thermodynamic effects, increase slightly above one with increasing supersonic flight speed. Potentially attractive and promising application areas of ejector processes lie in the field of aircraft-engine integration relevant to VSTOL, STOL, and vehicle boundary layer acceleration.
Steady flow ejector processes, as known today, are based on momentum exchange between two mass streams flowing in the same direction through a mixing duct. Hereafter such processes will be referred to as "coflowing momentum exchange processes." At the beginning of mixing, the two interacting gaseous media have differences in one or more of the following fluid flow parameters: velocity, total and/or static pressure, total and/or static temperature, and physical or chemical characteristics (chemical reactions during mixing not being considered). The medium having, at the onset of mixing, the greater total pressure is called the "primary medium" and the medium having the lower total pressure is called the "secondary medium."
Two fundamental characteristics of current steady coflowing ejector processes prevent this type of momentum exchange process from being applicable as the primary component process in an aeropropulsion system.
First, there are high intrinsic mixing losses in a steady coflowing ejector. The differences between the flow parameters (speed, pressure, temperature) of primary and secondary flow are largest at the beginning of mixing and equilibrate through the process of mixing to equal temperature, speed, and pressure. Thereby, the entropy of the mixture is increased over the sum of the entropies of the primary and secondary media prior to mixing. The greater the initial differences are between the flow parameters of primary and secondary working media, the greater is this total entropy increase. For example, consider a gas turbine engine. The differences in flight stagnation pressure and temperature (secondary conditions) and combustor exit stagnation pressure and temperature (primary conditions) are so large that the mixing losses in a coflowing ejector would greatly exceed the losses in corresponding turbomachinery. This would be true even for an ideal without skin friction and diffuser losses, and with supersonic flow after mixing.
Second, there is a inherent limitation of the amount of energy that can be transferred from the primary to the secondary working medium in a steady coflowing ejector. In a steady coflowing process the primary and secondary working media are brought by mixing to a uniform speed, V.sub.m, total pressure, P.sub.om, and total temperature, T.sub.om. Since P.sub.om and T.sub.om are different from the stagnation conditions P.sub.os and T.sub.os of the secondary working medium prior to mixing (P.sub.os and T.sub.os correspond to the level of zero availability), it follows that availability is left after mixing. This in turn means that in the coflowing ejector only a fraction of the available energy of the primary working medium can be transferred to the secondary working medium.
Assuming that it is possible to have self-sustained operation of a momentum exchanger, it is important to understand the interface stability between two swirling flows. Reference is made to the text entitled Boundary--Layer Theory by Dr. Hermann Schlichting, Sixth Ed. (translated), published by McGraw-Hill Book Company, New York NY (1968), particularly pages 500-503 referring to the work of G. I. Taylor, and to the text Jets, Wakes, and Cavities by G. Birkhoff and E. H. Zarantonello, published by Academic Press Inc., New York NY (1957), particularly pages 251-255 and the discussion of Taylor instability as observed by Sir Geoffrey Taylor.
FIG. 2 of the drawings shows the interface between two concentric rotating cylindrical flows: the inner flow (subscript i hereinafter) has density .rho..sub.i and velocity U.sub.i and the outer flow (subscript o hereafter) has density .rho..sub.o and velocity U.sub.o. As shown on FIG. 2, there are four significant conditions which are termed stable, semi-stable, semi-unstable, and unstable. Each of these is explained below.
Case 1: .rho..sub.i &lt;.rho..sub.o and .rho..sub.i U.sup.2.sub.t,i (r')&lt;.rho..sub.o U.sup.2.sub.t,o (r') PA0 Case 2: .rho..sub.i &gt;.rho..sub.o and P.sub.i U.sub.i.sup.2 &lt;.rho..sub.o U.sub.o.sup.2 PA0 Case 3: .rho..sub.i &lt;.rho..sub.o ; .rho..sub.i U.sub.i.sup.2 &gt;.rho..sub.o U.sub.o.sup.2 PA0 Case 4: .rho..sub.i &gt;.rho..sub.o and .rho..sub.i U.sub.i.sup.2 &gt;.rho..sub.o U.sub.o.sup.2
Under these conditions the interface is initially stable and remains stable after the velocities equilibrate. A distinction can also be made between the following velocity conditions:
U.sub.o =U.sub.i : This is the most stable condition (it corresponds to an inversion layer in meteorology). PA1 U.sub.o &lt;U.sub.i : Wave perturbations resulting from the velocity difference at the interface, transfer momentum from the inner to the outer swirl. PA1 U.sub.o &gt;U.sub.i : Wave perturbations at the interface, transfer momentum from the outer to the inner swirl.
Initially the two swirls are Taylor stable at the interface at the radius (r'). However, since U.sub.o must be greater than U.sub.i in order to satisfy the above given initial conditions, momentum is transferred from the low density outer flow to the high density inner flow. As the inner velocity increases eventually a point is reached where the flow is unstable since .rho..sub.i &gt;.rho..sub.o.
Due to the fact that initially the two swirls are Taylor stable and later become unstable, this flow is called "semi-stable." The reorganization into the end condition requires a much longer time than those cases where initially the two swirls are Taylor unstable.
The two swirl flows are initially Taylor unstable, and therefore the interface disrupts immediately, and large eddies of high velocity low density mass enter into the outer swirl of high mass density and lower velocity. Velocity equilibration is quickly reached and the flow density eddies are driven back toward the interface by buoyancy forces. This momentum exchange process is very intense, while irreversible mixing is slight. In this case the flow is initially unstable but becomes stable, therefore this process of momentum exchange is termed "semi-unstable."
The two swirl flows are unstable. The inner swirl having the larger density medium and the larger total pressure disrupts the interface and will only be stable when it moves to the outside, while the outer medium having the lower density and the lower total pressure seeks the inner core and is stable when it moves to the inside.